For extracellular recording, olfactory Q-VD(OMe)-OPh were immobilized on the microelectrode surface of MEA chip as described above. Basically, extracellular recording of olfactory cells by microelectrodes is based on the mechanism of cell-microelectrode electric coupling (Fig. S3). Olfactory cells can generate membrane potential changes under odorant stimulations due to the transmembrane current produced by the ion channels located on the cell membranes. The cell membrane potential changes can polarize the microelectrode via electric field, which can then lead to the redistribution of charges at the interface between electrode and electrolyte. The charge changes will result in the voltage changes of microelectrode. This can be used as the output signals of microelectrodes for recording, amplification, and processing. Based on this mechanism, the responses of olfactory cells to various odorant stimulations were recorded by the extracellular recording of microelectrodes. For the data processing, extracellular recording signals from olfactory cells when no odorant was applied were taken as the control signal. The mean value of the control signal was used to quantify the baseline noise and extract the cell spikes based on threshold method. In order to minimize the error spokes and undetected rate, we optimized the threshold for spike extraction, which is 1.5 times of mean value of control signal. All the odorants used in this study were purchased from Sigma–Aldrich (USA) and dissolved in the cell culture media and diluted to the desired concentration to serve as odorant stimulations.